Newborns differ a great deal in terms of their size at birth. In particular, their weight, length and head circumference are determined by how well they have grown in the womb and their age at birth, i.e. premature babies are smaller than babies born at term. We seek to understand whether these differences at birth influence the chances of developing infections during childhood. The reason we are asking the question is that our immune system, which helps us fight infections, starts to develop as we are growing inside our mother’s womb. When we are born, we get exposed to different microbes, which train our immune system to fight the infections they cause. The vaccines that babies receive also do this. If a baby’s immune system is poorly developed at birth, for example because of premature delivery, it may take longer for the infant to acquire immunity or their immune system may simply never be as strong. As these infections are quite rare, we need access to large databases of health records, such as IORD, to answer these questions.